The office environment has, for many years, been the site of objectionable noise generators, namely, typewriters and high speed impact printers. Where several such machines are placed together in a single room, the cumulative noise pollution is disturbing and may even be harmful to the health and well being of the machine operators and other occupants. The situation is well recognized and has been addressed by governmental bodies as well as by the technical community. Legislative and regulator bodies have set standards for maximum acceptable noise levels in office environments. In response to the call for a more healthful office environment, attempts have been made to reduce the noise by several methods: enclosed impact printers in sound attenuating covers; designing impact printers in which the impact noise is reduced; and designing quieter printers based on non-impact technologies, such as ink jet and thermal transfer.
Typically, conventional impact printers generate an average noise in the range of 70 to just over 80 dBA, which is deemed to be intrusive. When reduced to the 60-70 dBA range, the noise is construed to be objectionable. Further reduction of the impact noise level to the 50-60 dBA range would improve the designation to annoying. Clearly, it would be desireable to reduce the impact noise to a dBA value in the low to mid-40's. This represents a very aggressive dropoff in printer impact noise. Loudness levels measured on a dBA scale represent human perceived levels of loudness as opposed to absolute values of sound intensity. When considering sound energy represented in dBA (or dB) units, it should be borne in mind that the scale is logarithmic and that a 10 dB difference means a factor 10, a 20 dB difference means a factor of 100, a 30 dB difference means a factor of 1000, and so on.
It should be apparent that the printing noise referred to is of an impulse character and is primarily produced as the hammer impacts and drives the type character elements against the ribbon, the print sheet and the platen with sufficient force to release the ink from the ribbon. This impact noise masks other noises in the system. However, once the impact noise has been substantially reduced, the other noises associated with the operation of the printer will no longer be extraneous. Therefore, the design of a truly quiet printer requires the designer to reduce all other noise sources, such as those arising from carriage motion, character selection, ribbon lift and advance as well as from miscellaneous clutches, solenoids, motors and switches.
A quiet printer is disclosed in U.S. Ser. No. 751,169 filed on July 2, 1985 in the name of Andrew Gabor and entitled "Quiet Impact Printer". This application is assigned to the same assignee as the present invention. In the impact printer of that invention, a heavy mass is set in motion to accumulate momentum, for delivery to the platen by a movable hammer, or print tip, through a suitable linkage. The print tip drives a selected character pad of a printwheel across a throat distance, from its home position, against an inke releasing ribbon and then to the surface of the image receptor sheet, held adjacent to a platen. The entire excursion of the driven character pad includes an accurately controlled rapid pre-contact movement, through a throat distance of about 50 mils, and then a postcontact deformation, or penetration movement of about 5 mils. The contact velocity must be kept low for inherently quieter operation. In fact, the velocity of the print tip may be substantially arrested immediately prior to contact with the platen.
In order to accurately establish the pre-contact velocity profile, the throat distance must be accurately controlled. This is accomplished by providing a stationary reference position surface, on the reciprocating carriage, strategically located with respect to the printwheel and the platen. As the printwheel spokes pass against this reference surface they will be moved toward the platen to establish the proper throat distance.
It should be noted the U.S. Pat. No. 3,840,105 (Kittredge) entitled "Guide and Alignment Member" discloses a wire form member which prevents interference between out-of-line printwheel spokes and the hammer. The wire also serves to position the spokes at a given reference position before impact is achieved, for controlling the impact energy. As the rapidly rotating printwheel spokes pass against the alignment member a "picket fence" noise is generated. This phenomenon is analogous to the noise generated by rapidly moving a stick along a picket fence. However, since the Kittredge printer is no doubt a conventional impact printer, circa 1973, the "picket fence" noise would not be obtrusive as it would have been masked by the other noises, primarily the impact noise.
The printwheel of the present invention is to be used in a quiet printer, wherein the impact noise has been substantially reduced. In such a printer, other sources of noise such as the "picket fence" noise, become appreciable and objectionable. Therefore, it is the primary object of the present invention to eliminate the "picket fence" noise by suitably modifying the printwheel.
It is a further object of this invention to eliminate wearing of the printwheel spokes as they pass over the reference position surface.